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<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
      var PI = "3.14159265358"
      if(true){ 
          //代码块内部，暂时性不能使用PI
          console.log(PI);
          const PI = "123"
          //这里以后就可以使用PI 打印出123
      }
      console.log(PI)//3.14159265358
    </script>
</body>
</html>